1. Field of the Invention
The present invention relates mainly to an electromagnetic pickup device of an electronic piano, and particularly to an electromagnetic pickup device in which pole pieces are made so as to eliminate relative amplitude level difference between odd- and even-numbered higher harmonics.
2. Description of the Prior Art
There has been well known in the art a pickup device of an electronic piano in which vibratory reeds or the like are knocked by hammers to produce mechanical vibration of the reeds which is converted into electric signal by pickups disposed opposite to the reeds for picking the vibration. The construction of this pickup device relating to its electrodes or pole pieces is disclosed in detail in, for example, U.S. Pat. No. 3,038,363.
As the pickup it is generally known to use an electromagnetic pickup or an electrostatic pickup. The construction of a normally used electromagnetic pickup is publicly known as shown in FIG. 1.
In FIG. 1, substantially a U-shaped bed 1 having a large mass is provided with a plurality of cantilever-type vibratory reeds 2 made of a magnetically permeable material having various lengths corresponding to respective musical scales. Opposite to the free end of each reed is provided a pole piece 3 with a proper gap therebetween which is axially inserted into the center of a bobbin 4. The bobbin 4 has bored therein at its upper end a recess 5 in which there is placed a magnet 6 that is magnetized as illustrated. The bobbin 4 is also wound thereabout with a winding 8. An L-shaped metal piece 7 is secured at its one side to the bed 1 with the bobbin 4 being fixed thereto at its other side. With such an arrangement, when one of the reeds 2 is knocked by a hammer 9, the reed 2 is mechanically vibrated due to cross magnetic flux produced from the pole piece 3 so that the winding 8 induces an electric signal which corresponds to the mechanical vibration of the reed. An equivalent circuit having the pole piece arrangement of FIG. 1 is shown in FIG. 2, in which an axial gap between pole piece 3 and reed 2 is taken as l and an axis deviation therebetween as b. Assumed that the reed tip makes a sine-wave vibration with amplitude A and frequency f, magnetic resistance R is shown as R=K[l.sup.2 +(b+A cos 2.pi.ft).sup.2 ] and magnetic flux .phi. is shown as .phi.=F/R where F is a magnetomotive force produced by the permanent magnet 6. Taking the number of winding as n, a voltage e induced in the winding 8 is expressed as the following formula (1) based on e=-n(d.phi./dt): ##EQU1## where "K" is a proportional constant and "t" is time. As shown in FIG. 3, the signal voltage e is a periodic function of period T(=1/f) so that it exhibits a waveform which is symmetric with respect to a point of T/2. A rising-up period T.sub.o is given as follows: ##EQU2## If the deviation b approaches a zero, T.sub.o =T/4 is obtained resulting in a vibrating waveform having a period of T/2 or a frequency of twice the fundamental frequency f. With the electromagnetic pickup having the above construction, the waveform of FIG. 3 is spectrally distributed as shown in FIGS. 4A, 4B and 4C.
In FIGS. 4A to 4C, abscissa represents higher harmonic number or order n and ordinate represents relative amplitude level. FIG. 4A shows a spectral distribution in a case of large deviation b between reed 2 and pole piece 3, FIG. 4B that in a case of small deviation b, and FIG. 4C that in a case of zero deviation, respectively. The sound pitch of an electronic piano is determined by the relation among pitches of the fundamental and higher harmonic components (the order number 1 of harmonic is called as a fundamental tone and those 2, 3 . . . of harmonic (as harmonics) in the spectral distribution of FIGS. 4A to 4C according to the voltage waveform of FIG. 3). The sound pitch is dependent on the fundamental frequency or fundamental tone. Even when the fundamental tone is absent, the difference between harmonics, such as 3f-2f, is heard corresponding to the fundamental tone.
In the prior art electromagnetic pickup device, as shown in FIGS. 4A to 4C, the relative amplitude levels of odd higher harmonic numbers 1, 3, 5 . . . are very small or zero as compared with those of even higher harmonic numbers 2, 4, 6 . . . . As a result, in the cases of FIGS. 4A and 4B a difference tone between even harmonic components (a pitch of higher octave corresponding to double the fundamental frequency, for example, 4f-2f) is heard stronger than a different tone between even and odd harmonic components (a pitch corresponding to the fundamental tone) so that sound pitch becomes quite uncertain. In the case of FIG. 4C, since there is no odd harmonic component, the sound is heard positively as a higher octave. Therefore, in the prior art pickup of FIG. 1 the pitch becomes very uncertain which causes poor selection of tone or improper feeling of sound intervals.
The above-cited U.S. patent has also disclosed an electromagnetic pickup which detects flux variation of only half cycle of the vibratory reed 2 in order to eliminate the above described defect. This construction is shown in FIG. 5 in which elements corresponding to those in FIG. 1 are indicated by the same reference numerals with their repeated description being omitted. In FIG. 5, the pole piece 3 is fixed directly to the bed 1 and wound thereabout with the winding 8, while the vibratory reed 2 is magnetized as illustrated and mounted on the bed 1 in a cantilever type. A voltage waveform derived from the winding 8 by vibrating the reed 2 knocked with a hammer is shown in FIG. 6 and its spectral distribution is shown in FIG. 7. The spectral distribution in this case is reverse to that of FIG. 4B. In other words, the fundamental tone is the highest and even harmonics (2, 4, 6 . . . ) are relatively low as compared with odd harmonics, but the fundamental tone is always present so that sound pitch will not become uncertain as shown in FIG. 4. Meanwhile, as shown in FIG. 6 no voltage is generated during the half cycles from 0 to T/4 and from 3T/4 to T so that its detection efficiency becomes very small and it is impossible to change the form of spectrum by adjusting the deviation b between pole piece 3 and reed 2 as shown in FIGS. 4A to 4C with the result that only a simple sound is defectively obtained.
The above U.S. patent has also disclosed another type of pickup in which as shown in FIG. 8 the reed 2 and electrode 3 are coupled in a electro-static manner and the mechanical vibration of the reed is detected as the variation of capacitance therebetween. In this case, the electrode 3 is shaped corresponding to the path of reed tip in the case when the reed 2 is knocked by the hammer 9.
The above-described pickup detects an output signal whose waveform is shown in FIG. 9 and whose spectral distribution is shown in FIG. 10, respectively. As will be apparent from this spectral distribution, the uncertainty of sound pitch will not appear similarly as in the case of FIG. 7. However, this electrode construction is of a static pickup type, so that if this pickup was replaced by the electromagnetic pickup as shown in FIG. 1 or FIG. 5, the same spectral distribution as described in FIG. 7 would be obtained resulting in the same defect as the prior art pickup of FIG. 5. The measured result of the above is shown in FIG. 18-H.